The noun dissimulation describes the act of faking your true feelings. Your dissimulation of happiness might fool strangers but your close friends can tell it's all an act.
The word dissimulation implies that the wool is being pulled over someone's eyes, or they're being fooled or tricked by someone's deceit. The origin of dissimulation is the Latin dissimulationem, which means "a disguising or concealment." Don't confuse dissimulation with dissimilation, which describes the act of making something different or less similar.